Our description of the UGT1 locus has proven critical for the determination of genetic defects in patients with Crigler-Najjar(CN) diseases. The original version of the gene complex (UGT1A-UGT1F) codes for 2 bilirubin (Br), 3 Br-like, and 1 phenol transferases. In the 5' region, six different exons 1, each with an upstream promoter and each encoding the amino terminus of an isoform, are arrayed in series with 4 common exons encoding six identical carboxyl termini in the 3~ region of the locus. By way of extending the locus seven cosmid clones with at least six additional exons 1 are being mapped for restriction endonucleases and analyzed by Southern blot hybridization. The six exons have been sequenced and tentatively arranged in the series. The newly identified exons are related to the unique region of a second class of phenol isoforms typified by HLUG P4 that preferentially metabolizes bulky phenols. Out of 13 different deleterious mutations in the UGT1A gene of CN patients, 4 missense ones in CN-I and 4 in CN-II patients proved to either completely inactivate or partially inactivate HUG-Br1, respectively, accounting for the CN-II phenotype. A deleterious phenylalanine codon-180 deletion mutant (CN-I) illuminated a conserved hydrophobic Micro-region A (MR A, codons 161-180) which, according to computer analysis, is a buried hydrophobic helical structure predicted to be associated with membrane and is unique to the bilirubin isoform. A missense mutation, G276, in a CN-I patient disrupted a strictly conserved di-glycine (codons 276/277) in a conserved Micro-region B (MR B, codons 270-288) in a region predicted to form a beta-hair pin loop; the mutation completely abolished activity. The conservation of critical residues and MR-B between HUG-Br1 and the soluble but related baculoviral UDP-glucosyltransferase (egt) support the claim that the viral protein could serve as a structural model for the mammalian bilirubin isoform. Further, two different missense mutations in one allele of the acetaminophen-selecting UGT1F-encoded HLUG P1 were uncovered in the heterozygous state of genomic DNA of an apparent unaffected individual, promising to be the first defect uncovered at a transferase locus that affects drug metabolism.